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The H II region created by the progenitor of SN 1987A was further heated andionized by the supernova flash. Prior to the flash, the temperature of the gaswas 4000 - 5000 K, and helium was neutral, while the post-flash temperature wasonly slightly less than 10^5 K, with the gas being ionized to helium-likeionization stages of C, N and O. We have followed the slow post-flash coolingand recombination of the gas, as well as its line emission, and find that thestrongest lines are N V 1240 and O VI 1034. Both these lines are good probesfor the density of the gas, and suitable instruments to detect the lines areSTIS on HST and FUSE, respectively. Other lines which may be detectable are NIV] 1486 and [O III] 5007, though they are expected to be substantially weaker.The relative strength of the oxygen lines is found to be a good tracer of thecolor temperature of the supernova flash. From previous observations, we putlimits on the hydrogen density, n_H, of the H II region. The early N V 1240flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}cm^{-3}, where \eta is the filling factor of the gas. The recently reportedemission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman& Burrows (1992). For the more energetic 500full2 burst the density is n_H =(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models ofthe X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seemsplausible that the observed [O III] emission is produced primarily elsewherethan in the H II region. We also discuss the type of progenitor consistent withthe H II region. In particular, it seems unlikely that its spectral type wasmuch earlier than B2 Ia.Comment: LaTeX, 23 pages including 4 figures. To appear in ApJ (Main Journal

Flash Ionization of the Partially Ionized Wind of the Progenitor of SN 1987A